Frequency modulated picture receiver



Dec. 26, 1950 A. THOMPSON 2,535,611

FREQUENCY MODULATED PICTURE RECEIVER Original Filed April 22, 1944 I E l6' INVENTOR. L 00/6 A. T/mMArmv BY M Patented Dec. 26, 1950 UNITEDSTATES PATENT OFFICE FREQUENCY MODULATED PICTURE RECEIVER Louis A.Thompson, Rocky River, Ohio, assignor to NEA Service, Inc., Cleveland,Ohio, a corporation of Delaware Claims.

This invention relates to improvements in electrical circuits and moreparticularly to methods and apparatus that are especially useful in thetransmission of intelligence by wire or radio. This application is adivision of my copending application, Ser. No. 532,252, filed April 22,1944, since issued as Patent No. 2,488,517.

It has become common practice to transmit pictures electrically by meansof picture transmitters which are connected to each other by telephonelines or the like. A machine that is designed to transmit pictureseflicic-ntly over a telephone line may not easily be adapted to transmitpictures by radio and one adapted for radio transmission may not easilybe adapted to transmit over a telephone line or the like.

It is well known that pictures, in order to be reproducible withfidelity when transmitted by wire, rely to a considerable extent on theperfection of the line connection between the transmitter and thereceiver. That is, during transmission, variation in signal, line noisesand other disturbances are usually apparent in the final picture and, insome instances, have such effect thereon as to make the pictureillegible. It is particularly desirable in picture transmission fornewspaper reproduction that as good a picture as possible be obtainedbecause other losses occur during the reproduction which, if coupledwith the losses taken during transmission, may make the picture so poorthat it cannot be used.

By my present invention I have provided an improved method and apparatusfor the transmission and reception of pictures either over the wire orrad o whereby the picture is substantially unaffected by line noises,changes in volume and in the case of radio transmission, by static,fading and the like.

In carrying out the invention, I contemplate using a standard picturetransmitter and a standard picture receiver, and the apparatus about tobe described herein comprises an inverter for use during reception.However, it should be borne in mind that the certain parts of theinverter could be built into the receiver to provide a unitary device.Therefore, the invention in its broader aspects contemplates a newmethod of picture reception and in its more specific aspects, a meansfor converting existing machines so that the new method of reception canbe effected thereby.

A brief description of the function of the apparatus will assist in anunderstanding of the apparatus itse f here nafter describedspecifically.

To begin with, the picture transmitter may be 2 of the characterdescribed in my Patent No. 2,- 284,027 of May 26, 1942. As described inthe above patent, it contemplates scanning a picture to provide a signalin the output of the machine which may be an 1800 cycle signal thatvaries in strength or amplitude in accordance with the reflection oflight from the picture. This may also be considered for the purpose ofexplanation as an 1800 cycle amplitude modulated signal, the

amplitude modulations of which carry the intelligence. Ordinarily, thissignal is sent from the picture transmitter to the picture receiver overthe telephone lines or by means of land line connections. In thereceiver the signal issued to reprcduce the picture. During the processof transmission, an amplitude modulated signal sometimes accumulatesundesirable components due to line noises or increases and decreases ins gnal strength, all of which affects the desired modulation componentand which results in defects in the finished picture.

"Therefore, instead of sending the signal out on the lines in the formof an amplitude modulated wave, as it comes from the transmitter, I passit through a converter as described in my aforementioned copendingapplication. In the converter I amplify the signal to a predeterminedlevel. Next, the si nal is demodulated. Then I utilize the demodulatedsignal to control a variab e frequency oscillator. The variablefrequency oscillator and a fixed freduency oscillator are each coupledto a mixer demodulator which detects the difference between the twooscillators and wherein the signal is again demodulated to provide afrequency modulaed audio signal. This signal is then amplified and appers in the output of the converter. All of the foregoing is effected inthe converter system, and the output may now be used to modulate anykind of a transmitter or may be sent by wire if desired without anychange. In the case a transmitter is modulated, it may be either aso-called amp itude modulated type transmitter, or it may be a frequencymodulated type of transmitter.

The signal, after being transmitted by radio, is received by anysuitable type of receiver, depending on the final type of tran mitterused. Dur-'- ing the transmission, it may be effected in many ways. fadng, all of which under ordinary circumstances m ght be sufficient torender the ordinary :amplitude modulated signal substantially uselessfor hi h fidelitv reception of picture.

After being received, it is amplified together with the above mentionedundesirable defects and It may accumulate static, noise and/0r thenpasses through the inverter. In the inverter it passes through a cascadelimiter which very greatly limits the signal, taking off all amplitudemodulations. Next, it is reconverted to an amplitude-frequency modulatedhybrid signal, then rectified and passed through a filter. Then thissignal is combined with an 1800 cycle carrier. The signal has now becomean 1800 cycle constant frequency, amplitude modulated signal which maybe supplied to the standard picture receiver with a result, that thepicture is substantially the same as it would have been had thefacsimile transmitter been directly connected to the facsimile receiver.Thus the signal is handled in such a manner that the usual losses whichoccur in transmission do not aifect it.

It is therefore among the objects of my invention to provide anapparatus whereby a picture signal, such as is received by radiotransmission, may be inverted to enab'e a conventional picture receiverto reproduce the same.

Another object of my invention is to provide an improved method ofreceiving pictures or intelligence which is substantially unaffected bystatic, fading or the like.

Another object of my invention is the provision of an improved frequencymodulation system and method of operation thereof.

Another object of my invention constitutes the provision of an improvedmethod and apparatus for inverting a signal.

Other objects of the invention are the improvements of circuits embodiedin the invention and includes an improved limiter circuit, an improveddemodulation circuit and an improved signal reconstruction circuit.

Still other objects of the invention and the invention itself willbecome more apparent from the following description of a specificembodiment of the apparatus used to receive a signal which apparatus isillustrated by the accompanying drawings and forms a part of thisspecification.

Fig. 1 is a schematic drawing of a complete receiving system forreceiving and reproducing the signals from the apparatus of my copendingapplication.

Fig. 2 is a simplified view of a portion of the circuit of Fig. 1.

Referring now to the drawings, throughout which like parts aredesignated by like reference characters: The receiving end of the systemis best shown in Fig. 1. Here the signals, if the transmission is byradio, are received over a suitable receiver 50. Connected to thereceiver 50 is an amplifier 5| in which the signal is raised to a highvalue. At the output to the receiver the signal may contain certainundesirable components such as noise and static that may have beenpicked up during transmission, and in addition, the signal may besubject to severe fading. In other respects, however, the signal may bethe same as it was when it was used to modulate the transmitter, i. e.,a frequency moduated signal having a swing between 1000 and 2500 cycles.

Next, the signal is put throu h what I prefer to term a push-pullcascade limiter circuit. This is effected by feeding the si nal into theprimary of a push-pull transformer 52, the secondary of which has eachopposite ends connected through the resi tances 5.3 and to the grids and4-3 of the tubes 55 and 55'.

The signal is built up to a hi h level to enable the limiter to functioneven in the presence of deep fading of the signal.

Each limiter tube may include a twin triode modulatitll Omponent.

and the connections are such that each triode section is connected incascade. The operation may best be understood by reference to thediagram in Fig. 2 which shows one-half of the pushpul limiter circuitwith the twin triode sections separated. Assuming that a fairly strongsignal is being received, this signal is amplified to a high level andis applied to the grid 55 of the first triode section of tlte tube 55through a series resistance 53. The grid 5 is driven positive on thepositive half of the signal which causes grid current to flow in thegrid-cathode circuit. This grid current causes an IR drop across theresistor 53 with the result that the signal into the tube is limited.

The anode voltage for the first triode section is supplied by the mannerin which the second triode section is connected. The anode 51 of thesecond triode is connected to the positive plate voltage supply, andnormally this tube draws current from the plate circuit to the cathode59 which is connected through a resistor 58 to the grid 60 of the secondsection and the anode 51 of the first triode section. The circuit thenfor the cathode of the second section is through the first triodesection to the cathode 56 and thence to ground. The voltage drop acrossresistor 58 provides bias for the grid 60. Since the cathode 56 of thefirst section is at ground potential, the plate voltage at 51 is aboveground potential due to the current flow in 58.

IVhen the positive signal appears on the grid 54, the plate 5? drawscurrent which is through the resistor 53, thus increasing the negativebias on the grid 58, which action decreases the normal plate current inthe second section thus re ducing the plate voltage at 57.

This still further increases the limiting action of the circuit. Thislimiting action is relatively large, depending, of course, on theamplitude of the signal. The final result is that the resultant waveform is devoid of all amplitude modulations caused by static, noise orother amplitude icked up previously in the system and is substantially asquare wave. Of course, on the other half of the Wave, the other limitertube 55' is operating in a like manner.

The maximum limiting of the signal voltage may be as much as from 2000to 2. In actual practice I have found that a desirable method ofoperation is to raise the signal to a relatively high value before it islimited thus providing sufficient signal to always provide limitingaction. In practice I have found it desirable for the limiter to startfunctioning on voltages as low as 2 volts.

As a result of the limiting action, the output from the limiter issubstantially a constant voltage of varying frequency.

The output from the two limiter circuits are connected in push-pull tothe primary of the transformer 5i the secondary of which feeds into asloping filter 62. This filter, as the name implies, has a 45 degreesloping characteristic. A signal having a frrquency of 1000 cycles,which may be picture wl ite, will pas through the filter without anysubstantial loss. As the frequency of the signal increases the lossbecomes greater, hence a 2500 cycle signal suffers a very substantialloss. Likewise, the signals between 1000 and 2500 cycles sufier a losssubstantially in direct proportion to their frequencies. The result isthat the signal in the output of the sloping filter 52 comprises asignal that varies in amplitude in a mann r correspon ing to te originalam litude The signal at this point comprises a hybrid signal whichconsists of frequency and amplitude modulations.

The signal is then fed through the transformer 63 the secondary of whichis connected to the amplifier tube 64 which may be another twin triodewith the two sections connected in parallel, and in which the signal isamplifier.

The output from the tube 54 is connected to the transformer 65, thesecondary of which is connected to the anodes of the demodulator tube61. This tube rectifies both halves of the signal which then appears onthe cathodes of the tube which are connected together, and is apulsating direct current the amplitude of which varies according to themodulation envelope. The pulsations in the signal are removed in thefilter comprising the input condenser 68, the choke 69 and followed bythe series choke and condenser ill-l i connected between the conducto l2and ground. Thus at 73 there remains only a signal which correspondsexactly to the modulation envelope of the original signal.

The next step in the procedure is to use this signal energy toreconstruct an 1800 cycle amplified modulated signal in order that itmay be supplied to a conventional facsimile receiver and there used toreproduce the picture.

For generating the 1800 cycle carrier signal, I provide an 1800 cycleaudio oscillator 89 which may be of any standard type giving a good sinewave and having a constant frequency output of 1800 cycles. The outputfrom the oscillator is connected in push-pull to the grids M of the tube82, which may be a twin triode. The anode voltage for this tube isobtained from the signal (at 73) after it leaves the filter, the signalbeing supplied to the anodes through resonant filters each of whichcomprises an inductance 84 and 33 paralleled by capacities 85 and 81respectively. The two inductances are connected end to end and as statedeach has a condenser across it. The signal is fed to the junction of theinductances and capacities. The leads 88 and 6E; connect to the anodesof the tube 82. The two resonant filters B l-85 and 36-81 are tuned tothe frequency of the oscillato 80. Tube 82, which may be termed arestorer tube, only operates to pass a signal when plate voltage issupplied to the anodes. Because the plate voltages on this tube arebeing supplied by the signal which is substantially a replica of theoriginal signal us d to modulate the carrier at the facsimiletransmitter, the signal in the plate circuit of the tube varies with theplate voltage. The signal as it appears across the parallel resonantcircuits is an 1800 cycle amplitude modulated carrier which is suppliedthrough the coupling condensers 90 to the potentiometers 9! which areconnected in series and grounded at their junction. The sliders 92 ofthe potetiometers are connected to the control grids 93 of the twintriode tube 94 which operates in push-pull. The cathodes of the tube aregrounded through the resistor 95. The plate circuit of the tube isconnected to the output transformer 96. The tube 5 amplifies the signalwhich is thus supplied to the transfrrmer 95. The level of the signalmay be determined by a meter M indicator disposed in the output. Theoutput from the inverter may be connected directly to a standard picturereceiver it for reproduction or it may be sent over land wires to theplace where the picture receiver is located.

It should part'cularly be noted that the filter network ahead of thepoint 13 and comprising the inductance 69 and capacity 68, and theinductance 10 and capacity H are effective to prevent anything but theD. C. component of the desired signal from etting through to the point13, and that the resonant c'rcuits 84-85 and B6-81 are resonant to 1800and therefore present a high impedance to the 1800 cycle carrier andtherefore readily transfer the 1800 frequency through the couplingcondensers St to the input circuit of the tube 93. However, theseresonant circuits readily admit the D. C. energy from T3 to the anodesof tube 82.

Thus the signal in the restorer circuit is a clean cut signal withoutany spurious modulations due to the undesirable elements in the incomingsignal.

The two taps shown for the output meter provide means whereby when agreater resistance is in series, the gain can be raised and the properlevel determined for sending the picture by land wire. If the picture isfed directly to the picture receiver, the gain may be lowered and theproper level determined by shifting the meter to the tap where thelesser res'stance is in the circuit. In either case the meter enablesthe proper output to be determined. Having thus described my invention,I am aware that numerous and extensive departures may be made therefromwithout departing from the spirit or scope of the invention.

I clam:

l. A signal restoring system comprising a source of carrier frequency,an electron tube in the output of said frequency source and having apair of grids and a pair of anodes, said grids being coupled to saidfrequency source, a source of signal frequency and means connecting saidsignal frequency source to said anodes in said tube, said anode circuitincluding a pair of resonant circuits tuned to the frequency of saidcarrier frequency source, an amplifier, and means coupling said resonantanode circuits to the amplifier, said amplifier belng adapted to amplifysaid modulated carrier.

2. A modulation system comprising a source of carrier signal frequency,an electron tube in the output of said carrier source and having pairsof grds and anodes, said grids having connected thereto a source ofsignal and means for connecting said signal source to the anodes of saidtube, said signal connecting means including a filter comprising aseries resonant c;rcuit for preventing undesirable elements in saidsignal from passing therethrough, said anode circuit includ ng a pair ofresonant circuits tuned to the frequency of said carrier frequencysource and having a high impedance to said carrier frequency, anamplifier and means coupling said resonant anode circuits to theamplifier, said amplifier bein adapted to amplify said modulatedcarrier.

3. A modulation system comprising source of carrier signal frequency, anelectron tube h.av;ng a pair of control grids and a pair of anodes, saidcontrol grids being connected to the output of said carrier source, asignal source, means connecting said s gnal source to said anodes insaid tube including a series resonant circuit for bypassing undesirablesignals and transferring the desired signal, sa d anode circuitincluding a pair of resonant circuits tuned to the frequency of saidcarrier frequency source and having a high impedance to said carrier, anamplifier and means coupling sad resonant anode circuits to theamplifier, said ampiifier being adapted to amplify said signalstransferred by said resonant circuits.

ii A modulation system comprising a source of carrier frequency, anelectron tub-e having a pair of control grids and a pair of anodes, saidcontrol grids being connected to the output of said car rier source, asignal source and a series resonant circuit connecting said signalsource to the anodes in said tube, said series resonant circuit beingdesigned to eliminate undesired frequencies and providing a lowimpedance to frequencies in the range of the carrier frequency source,said anode circuit including a pair of parallel resonant circuits tunedto the frequency of said carrier irequency source, sa'd parallelresonant circuits providing a low impedance to said signals and a highimpedance to said carrier and said signals being transferred throughsaid parallel resonant circuits to said anodes, an amplifier and meanscoupling said resonant anode circuits to the amplifier, said amplifierbeing adapted to amplify sa'd signals in said resonant circuits.

5. A modulation system including a source of carrier frequency, anelectron tube having a pair of control grids and a pair of anodes, saidcontrol grids being connected to the output of said carrier source, as'gnal source and a series resonant circuit connectin said signal sourceto the anodes in said tube, said series resonant circuit being desgnedto eliminate undesired frequencies and providing a low impedance tofrequencies in the range of the carrier frequency source, said anodecircuit including a pair of parallel resonant crcuits tuned to thefrequency of said carrier frequency source, said parallel resonantcircuits providing a low impedance to the said signals and a highimpedance to said carrier and said signals being transferred throughsaid parallel resonant circuits to the anodes of said tube and providinga varying potent a1 thereon and said tube being adapted to amplify saidcarrier signal in accordance with the change in plate voltage to impresson the parallel resonant circuit an amplitude modulated wave, anamplifier and means coupl'ng said parallel circuits thereto, saidamplifier being adapted to amplify said composite signal from saidresonant circuits.

LOUIS A. THOMPSON.

Name Date Thompson Nov. 15, 1949 Number

